Offline computed and optimized pulse patterns may be used for modulating the semiconductor switches in an electrical converter. Based on the actual speed and flux reference (provided for example by an outer control loop), a controller may determine a best suited offline computed pulse pattern, which is then applied to the semiconductor switches of the electrical converter. Offline computed optimized pulse patterns allow the minimization of the overall current distortion for a given switching frequency.
When the electrical converter is used for driving an electrical machine, the current distortion is proportional to the harmonic losses in the stator winding of the electrical machine, while the switching frequency relates to the switching losses of the power inverter. In a grid-connected converter setting, stringent standards may be imposed on the voltage and current distortions a converter may inject into the grid. Traditionally, it has only been possible to use optimized pulse patterns in a modulator driven by a very slow control loop. This may lead to very long transients and to harmonic excursions of the currents when changing the operating point.
In EP 2 469 692 A1, a control method is proposed that combines the merits of direct torque control and optimized pulse patterns, by manipulating in real time the switching instants of the pre-computed optimized pulse patterns, no as to achieve fast closed-loop control. This so-called model predictive pulse pattern controller (MP3C) may address in a unified approach the tasks of the inner current control loop and modulator. MP3C may control a flux vector, which, in case of an electrical converter driving an electrical machine, is typically the stator flux linkage vector of the electrical machine. For grid-connected converters, the virtual converter flux may constitute the flux vector.
MP3C achieves short response times during transients, and a good rejection of disturbances. At steady-state operating conditions, due to the usage of optimized pulse patterns, a nearly optimal ratio of harmonic current distortions per switching frequency is obtained. Compared to state of the art trajectory controllers, MP3C may provide two advantages. First, a complicated observer structure to reconstruct the fundamental quantities may not be required. Instead, the flux space vector, which may be the controlled variable, may be estimated directly by sampling the currents and the DC-link voltage at regular sampling intervals. Second, by formulating an optimal control problem and using a receding horizon policy, the sensitivity to flux observer noise may be greatly reduced.
During transient operation, such as reference step or ramp changes, large disturbances and faults, the controlled variables, such as currents, electromagnetic torque and flux linkage, are typically required to change in a step-like fashion or they must follow a steep ramp. Examples include fast torque steps in high performance drives and power steps in low-voltage ride through operation.
In MP3C, closed-loop control is achieved by modifying the switching instants of the optimized pulse patterns' switching transitions in real time. More specifically, the switching transitions are modified in time, such that the flux error is removed at a future time instant. Note that in an optimized pulse patterns, the switching transitions are not evenly distributed in time. At very low switching frequencies, long time intervals may arise between two switching transitions. When a reference step is applied at the beginning of such a time interval, a significant amount of time may elapse before the controlled variables start to change, resulting in a long initial time delay and also prolonging the settling time.
Once the controlled variable has started to change, the transient response may be sluggish and significantly slower than when using deadbeat control or direct torque control, for example. The sluggish response is typically due to the absence of a suitable voltage vector that moves the controlled flux vector with the maximum speed and in the direction that ensures the fastest possible compensation of the torque or current error. In order to ensure a very fast transient response during a transient, at least one phase may need to be connected to the upper or lower DC-link rail of the converter. In a low-voltage ride through setting, for example, this may imply reversing the voltage in at least one phase from its maximal to its minimal value, or vice versa, during a major part of the transient.
Directly related to the behaviour of sluggish transient responses is the issue of current excursions during transients. Such excursions may occur when the switching transitions, which are to be shifted in time so as to remove the flux error, are spread over a long time interval. This may increase the risk that the flux vector is not moved along the shortest path from its current to its new desired position. Instead, the flux vector may temporarily deviate from this path, exceeding its nominal magnitude. This may be equivalent to a large current, which may result in an over-current trip.
Related issues may also arise at quasi steady-state operating conditions, where small variations in the operating point and/or the inverter voltages occur. Specifically, fluctuations in the DC-link voltage, resistive voltage drops in the machine's stator windings or in the grid impedance and transitions between different pulse patterns may lead to a degradation of the closed-loop performance of MP3C, if they are not properly accounted for.
Particularly in cases, where the optimized pulse patterns' switching transitions exhibit an uneven distribution in time and when a very low switching frequency is used, these flux errors may not be accounted for in a timely fashion due to the absence of suitable switching transitions. As a result, large flux errors may arise that build up and persist over significant periods of time, leading to a poor tracking of the optimized pulse pattern's optimal flux trajectory. This may impact the total harmonic distortion of the current in a detrimental way.
In “Direct torque controlling technique with synchronous optimized pulse pattern”, proceedings of the annual power electronics specialists conference. (pesc) Seattle, june 20-25, 1993; proceedings of the annual power electronics specialists conference (pesc), new York, IEEE, US, vol. Conf. 24, 20 Jun. 1993(1993-06-20) pages 245-250, XP010149065, DOI: 10.1109/PESC. 1993.471952 ISBN: 987-0-7803-1243-2, a generic method for direct torque control using synchronous optimized pulse pattern is disclosed.
Furthermore, “Model Predictive Pulse Pattern Control”, IEEE Transactions on Industry Applications IEEE Service Center, Piscataway, N.J., US, vol 48, no 2, 1 Mar. 2012 pages 663-676, XP011434186, ISSN: 0093.9994, DOI: 10.1109/TIA.2011.2181289 shows method for model predictive pulse pattern control that combines the optimal steady-state performance of optimized pulse patterns with the very fast dynamics of trajectory tracking control.
Moreover, in “Improved dynamic operation for direct flux control of active front ends with low switching frequency”, Power Electronics Specialists Conference, 2004. Pesc 04. 2004 IEEE 35th Annual Aachen, Germany 20-25 Jun. 2004, Piscataway, N.J., USA IEEE, US 20 Jun. 2004, ‘pages 3533-3539, XP010739481, ISBN: 987-0-7803-8399-9, a three-fold pulse pattern for direct flux control of a high-power multilevel active front ends (AFE) with low switching-frequency useful for reducing the harmonic content within the converter terminal voltage is mentioned.
Additionally, EP 1 717 941 A2 discloses a method for controlling a voltage source converter comprising a plurality of valves, each containing a plurality of extinguishable semiconducting elements in a high power application, wherein an executing pulse width modulation signal is provided for controlling the voltage source converter. The method comprises the following steps:                controlling the voltage source converter during a first period of time wherein the executing signal comprises a first pulse-width modulation signal, and controlling the voltage source converter during a second period of time, and        following the first period of time, wherein the executing signal comprises a second pulse-width modulation signal.        